Thermosensitive recording elements have wide application in the printing industry. For example, thermosensitive recording elements have been used in instruments for recording measurements, facsimiles, printers, thermal devices for computers, devices for preparing architectural and engineering drawings, and automatic vending machines for dispensing railroad tickets and luggage tags. In addition, thermosensitive recording elements have been used in devices for (a) printing thermal labels, (b) recording geophysical data, and (c) recording medical data.
Conventional thermosensitive recording elements generally comprise a support and a thermosensitive recording layer provided thereon. The thermosensitive recording layer primarily contains a binder, a substantially colorless electron donating dye precursor and an electron accepting compound also known as a developer. Heat is applied to the element by means of a thermal head, a thermal pen or laser beam. Upon heating, the dye precursor instantaneously reacts with the electron accepting compound to form a recorded image.
It has been found that defects in the imaging surface of thermosensitive recording elements can be easily obtained. For example, rubbing the element gently with hard objects such as paper clips, fingernails, and dirt particles or even folding the element can result in undesirable imprints in the imaging surface. This presents a barrier to their use in the printing industry.
Abrasion resistance can be greatly improved by separating the imaging components, i.e., by separating the colorless dye precursor from the developer. This is achieved by applying, for example, the developer containing coating composition to a support that is first coated with the dye precursor containing composition. However, due to the hydrophilic nature of the polymeric binders in the two component compositions, these coatings are subject to image development when stained with water.
Another way of improving abrasion resistance in thermosensitive recording elements is the use of high gloss coatings. The use of high gloss coatings is advantageous in preparing thermal printed labels, tags, or material for advertising. Such coatings can be produced by adding a top coating containing film forming polymers such as polyvinylalcohol or acrylic-methacrylic copolymers over the outermost layer, for example, the developer containing layer produced by the two coating process. However, polymer containing top coats such as these are subject to image development when stained with water. In addition, the low softening point of such polymers causes the element to adhere to the print head of thermal printers, and thus, images of poor quality are produced. The additional layer also adds to the cost of the thermosensitive element.
Improved abrasive resistance has also been achieved by the addition of a crosslinking agent to a dispersion containing a polymeric binder and developer. Dialdehydes, such as glyoxal, have been used as crosslinking agents and, along with small quantities of an acid catalyst such as p-toluenesulfonic acid (PTSA), have been added to the binder/developer dispersion. Although coating compositions such as these do produce improved abrasion resistance, there is a problem with the crosslinking agent prematurely crosslinking with the polymeric binder. Such premature crosslinking causes the coating to become viscous and thus the ability to coat becomes dependent upon the type of coating apparatus used by the operator. To compensate for this, either a different crosslinking agent must be used or the concentration of the crosslinking agent must be carefully controlled so that the viscosity of the coating remains in a workable range for application by the particular coating apparatus.
Accordingly, there is a need for a thermosensitive recording element that has improved abrasion resistance, and reduced adherence to the print head without the associated coating problems addressed above.